1. Field of the Invention
The present invention relates to a data transmission system which includes a plurality of transmitters connected to a transmission path of SDH (Synchronous Digital Hierarchy)/SONET (Synchronous Optical NETwork) in a ring configuration.
2. Description of the Related Art
RPR (Resilient Packet Ring) is drawing attention as a technology for increasing the availability of networks which are configured to conform to SDH/SONET. RPR is defined in 802.17 recommended by IEEE (Institute of Electrical and Electronic Engineers), and is capable of transmitting packets in a high bandwidth such as 2.4 Gbps, 10 Gbps or the like.
Referring to FIG. 1, a data transmission system disclosed in JP-2006-80888A is shown, where the system comprises RPR devices 90-94, layer-3 switch (hereinafter labeled “L3SW”) 96, and terminal 97.
RPR devices 90-94 are connected in a ring configuration through radio transmission path 95. Also, each of RPR devices 90-94 is connected to a plurality of terminals 97 through L3SW 96.
Radio transmission path 95 comprises full-duplex transmission lines, where each transmission line is comprised of eight channels in total, each of which having a bandwidth of 6 Mbps.
Terminal 97 is a terminal which has a MAC (Media Access Control) address or an IP (Internet Protocol) address. For example, terminal 97 may be a personal computer, an IP telephone or the like.
RPR device 91 forwards frames received on each channel (from channel 1 to channel 8) from RPR device 90 to RPR device 92. Also, RPR device 91 detects a fault based on the frame reception situation on each channel (from channel 1 to channel 8) from RPR device 90.
When the occurrence of faults is detected on one-half (four channels) or less of all channels (eight channels), RPR device 91 notifies RPR device 90, which is the source of the frames, of information on channels on which the faults have occurred. Upon receipt of the channel information from RPR device 91, RPR device 90 narrows down the bandwidth of radio transmission paths 95 of normal channels, and continues the transmission of frames.
When the occurrence of faults is detected on more than one-half (five channels) of all channels, RPR device 91 notifies RPR device 90 of non-permission information which indicates that communications are not permitted. Upon receipt of the non-permission information from RPR device 91, RPR device 90 transmits frames to RPR device 94 in a direction opposite to RPR device 91. This processing is called wrapping processing which is one of the protection functions defined by RPR.
In the data transmission system described above, faults can occur in a plurality of sections. For example, faults can occur in a section between RPR device 90 and RPR device 91, and in a section between RPR device 93 and RPR device 94. In such an event, if faults occur on a number of channels which exceed one-half of all channels in each section, protection processing (wrapping processing) is performed in each section to break the network into a plurality of fragments. In this event, the broken network can result in an inability to transmit frames returned by the wrapping processing.
In the following, a specific description will be given with reference to FIG. 2.
Here, a description will be given of exemplary operations when RPR device 93 transmits frames to RPR device 91 in the event that faults occur in section A between RPR device 90 and RPR device 91 and in section B between RPR device 92 and RPR device 93 in FIG. 2.
In a normal operation, RPR device 93 transmits frames to RPR device 91 through RPR device 90. When faults occur on a number of channels exceeding one-half of all channels in section A, protection processing (wrapping processing) is performed. As a result, RPR device 90 returns the frames from RPR device 93 back to RPR device 93. Then, RPR device 93 transmits the frames from RPR device 90 to RPR device 91 through RPR device 92. In a scenario where such an operation is performed to forward the frames, if faults occur in section B on a number of channels exceeding one-half of all channels, triggering protection processing (wrapping processing), RPR device 93 again returns the frames from RPR device 90 back to RPR device 90. As a result, the frames cannot be transmitted from RPR device 93 to RPR device 91.